1. Field of the Invention
The present invention relates to a fixed-position stop control apparatus for a rotation shaft that is used to stop a rotation shaft of a machine tool or the like at a desired position.
2. Description of the Related Art
In a fixed-position stop control for stopping a rotation shaft of a motor of a machine tool such as a computer numerically-controlled machine tool, a mechanical origin that becomes a basis when stopping is determined on a rotation shaft, and a reference point is also determined at a fixed side.
A fixed-position stop control apparatus for a rotation shaft performs a speed control, using one of a speed instruction generated by a higher level control apparatus and a predetermined speed instruction. When a desired stop position is generated as a stop-position instruction, a speed control of the rotation shaft is changed over to a position control of the rotation shaft. Thereafter, the rotation shaft stops at the stop position based on the position control.
FIG. 5 is a diagram showing a relationship between a current speed of a rotation shaft and time according to an related art as disclosed in Japanese Patent Unexamined Publication No. 6-195118, for example. As shown in FIG. 5, in a zone Z1, the rotation shaft is speed-controlled at a constant speed V0 according to an instruction from a higher level control apparatus. When the rotation shaft needs to be stopped (at time T1), the speed of the rotation shaft is gradually decreased to a predetermined speed VZ, as shown in a zone Z2. When the speed of the rotation shaft decreases to the speed VZ, the control of the rotation shaft is switched from the speed control (the zone Z2) to a position control (a zone Z3) at a time T2.
According to the position control of the related art, a total move amount until the rotation shaft stops is input in one operation as a stop-position instruction. The speed instruction of the rotation shaft in the zone Z3 is expressed using a position gain PG, as given by the following expression (1).“Speed instruction”=“PG”×“Position difference of the rotation shaft”  (1)
The speed instruction given by this expression (1) is drawn as an exponential curve which is convex downward relative to time.
However, usually, the speed instruction given by the above expression does not coincide with the predetermined speed VZ. Therefore, the speed changes rapidly between the zone Z2 and the zone Z3. Specifically, as shown by a broken line Y1 in a small zone Z31, a speed instruction that exceeds the acceleration and the deceleration ability of the rotation shaft is generated during a shift from the zone Z2 to the zone Z3.
Therefore, by setting a speed instruction upper limit, a speed instruction in the zone Z3 is expressed as given by the following expression (2).“Speed instruction”=min(“PG”“×”Position difference of the rotation shaft”, “Position instruction upper limit”)  (2)
With this arrangement, continuity of the speed instruction can be secured between the zone Z2 and the zone Z3.
When the expression (2) is used, the zone Z3 includes the small zone Z31 in which the speed instruction upper limit is employed, and a small zone Z32 in which “PG”×“Position difference of the rotation shaft” expressed by the exponential function is employed. In the small zone Z31, the speed of the rotation shaft corresponds to the speed instruction upper limit, and is, therefore, constant. This means that the acceleration and deceleration ability of the rotation shaft is not used in the small zone Z31. Because the small zone Z31 is present, the time required to reach a stop at the fixed position becomes long.
On the other hand, FIG. 6 shows a state that a speed control is changed to a position control while the rotation shaft is rotated at a relatively high speed, that is, a speed control (a zone Z2′) is changed over to a position control (a zone Z3′) at a speed VX (VZ<VX<V0). In this case, a speed instruction that exceeds the deceleration ability of the rotation shaft is generated in the zone Z3′ as shown in FIG. 6. Due to the foregoing, a relatively large mechanical shock occurs at the time of shifting from the zone Z2′ to the zone Z3′ (i.e., at the time of the switching from the speed control to the position control).
For the above reasons, in the related art, the predetermined speed VZ shown in FIG. 5 must be made relatively small. However, when the predetermined speed VZ is made small, the small zone Z31 is extended correspondingly, and the time required to reach a stop at the fixed position becomes long.
The present invention has been made in the light of the above situation. It is an object of the present invention to provide a fixed-position stop control apparatus for a rotation shaft capable of decreasing time required for a rotation shaft to stop at a fixed position.